Single frequency laser sources are of great interest for high bandwidth single-mode fiber communication systems when the data bit rate is limited by chromatic dispersion. High speed light output pulses can be generated by either direct modulation of the laser or continuous wave operation of the laser with an external modulator.
Longitudinal mode selection by a coupled cavity has been investigated as an approach to achieve single-longitudinal-mode operation of semiconductor lasers. (See D. A. Kleinman and P. P. Kisliuk, The Bell System Technical Journal, Volume 41, p. 453, March, 1962 and P. W. Smith, Proceedings of the IEEE, Volume 60, No. 4, p. 422, April, 1972.) The second cavity optically coupled to the laser can be either an air-spaced passive cavity or a laser cavity with bias-current control. (D. Renner and J. E. Carroll, Electronics Letters, Volume 15, No. 3, p. 73, Feb. 1979; C. Lin and C. A. Burrus, Post Deadline Paper No. PD5-1, Topical Meeting on Optical Fiber Communication, New Orleans, Feb. 28-Mar. 2, 1983; L. A. Coldren, B. I. Miller, K. Iga, and J. A. Rentschler, Applied Physics Letters, Volume 38, p. 315, Mar., 1981; and K. J. Ebeling, L. A. Coldren, B. I. Miller, and J. A. Rentschler, Applied Physics Letters, Volume 42, p. 6, Jan., 1983). A long laser cavity and a shorter second passive, or active, cavity are used in such a coupled cavity geometry. The undulation of the laser net gain spectrum in this compound resonator provides suppression of the unwanted laser longitudinal modes. The passive-active coupled cavity laser is attractive for its simplicity. The passive external cavity can be applied to existing lasers with certified designs. Use of a concave spherical mirror has been suggested by Preston et al to focus light to the laser for single mode operation. (See K. R. Preston, K. C. Woollard, and K. H. Cameron, Electronics Letters, Volume 17, No. 24, p. 931, Nov., 1981). However, manufacturing and alignment of the spherical mirror is difficult. A coupled planar Au mirror was shown by Lin et al to stabilize single-longitudinal-mode operation under high speed pulse modulation of a short cavity laser. (See C. A. Burrus, T. P. Lee, and A. G. Dentai, Electronics Letters, Volume 17, No. 25, p. 954, 1981 and T. P. Lee, C. A. Burrus, J. A. Copeland, A. G. Dentai, and D. Marcuse, IEEE Journal of Quantum Electronics, Volume QE-18, No. 7, July, 1982).
A short cavity laser, however, has been shown to have a short practical operation life time. (See K. Endo, S. Matsumoto, H. Kawano, T. Furuse, and I. Sakuma, Paper No. 954, National Convention of the Institute of Electronics and Communication Engineers of Japan (IECEJ), Sendai, Japan, April 2-4, 1983). A longer laser cannot be used with the flat mirror because of the poor optical coupling efficiency.